1. Field of the Invention
The present invention relates to an NOX sensor control apparatus (e.g., controller) which is connected to an NOX sensor and which controls the NOX sensor, and to a vehicle control apparatus (e.g., ECU) connected to the NOX sensor control apparatus.
2. Description of the Related Art
A known gas sensor for measuring the concentration of a specific gas component in the exhaust gas of an automobile includes at least one cell composed of a solid electrolyte member and a pair of electrodes provided thereon. The gas sensor is configured such that one electrode of the cell faces a measurement chamber into which the exhaust gas flows. In such a gas sensor, the gas concentration is measured based on an output from the electrode pair of the cell, and the measurement is stabilized by limiting (controlling) the amount of the gas which flows into the measurement chamber under a predetermined diffusion resistance. However, this conventional gas sensor poses a problem in that the amount of gas which flows into the measurement chamber changes in accordance with a change in pressure of the exhaust gas in the vicinity of the attached gas sensor. Further, the output changes even when the concentration of the specific gas remains unchanged, such that the measured gas concentration is subject to measurement error.
In view of the above, a technique has been developed for an air-fuel-ratio sensor which includes an internal space into which the gas to be measured is introduced under a predetermined diffusion resistance, and first and second electrochemical cells each composed of a solid electrolyte member and a pair of electrodes. According to this technique, when a sensor output y0 indicating oxygen concentration is corrected on the basis of the detected pressure Pg of the gas to be measured, a gas pressure variation index B, previously measured for an individual air-fuel-ratio sensor, is taken into consideration (see, for example, Patent Document 1).
In recent years, NOX sensors have been developed for detecting the NOX concentration of a to-be-measured gas by use of a solid electrolyte member. Further, an NOX sensor has been developed which can measure both NOX concentration and oxygen concentration (i.e., which can function as the above-described air-fuel-ratio sensor) and a control apparatus for controlling this type of sensor (see, for example, Patent Documents 2 and 3). In this NOX sensor, a to-be-measured gas is introduced into a first measurement chamber under a predetermined diffusion resistance, and the oxygen concentration of the to-be-measured gas is adjusted to a predetermined concentration by means of a first pumping cell composed of a solid electrolyte member and a pair of first electrodes. The to-be-measured gas having the adjusted oxygen concentration flows from the first measurement chamber into an NOX measurement chamber. Further, NOX contained in the to-be-measured gas is decomposed by means of a second pumping cell composed of a solid electrolyte member and a pair of second electrodes, whereby a second pumping current corresponding to the NOX concentration flows between the pair of second electrodes. In this manner, in the technique described in Patent Documents 2 and 3, the NOX concentration is calculated based on the second pumping current flowing through the second pumping cell, and the oxygen concentration is calculated based on the first pumping current flowing through the first pumping cell. Notably, in the technique disclosed in Patent Document 3, in order to correct the oxygen-concentration dependency of the second pumping current, the NOX concentration is finally determined in consideration of the first pumping current (oxygen concentration) and the second pumping current.
[Patent Document 1] Japanese Patent No. 2846735 (claim 2)
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 10-142194 (abstract, claim 1)
[Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 11-304758
3. Problems to Be Solved by the Invention
However, a technique has not yet been developed for correcting both NOX concentration information and oxygen concentration information obtained by use of an NOX senor, in accordance with the pressure of a to-be-measured gas. Further, the pressure dependency of the sensor output (first pumping current and second pumping current) varies among individual NOX sensors. Therefore, if a pressure-dependent variation in the sensor output or a concentration value calculated on the basis of the sensor output is corrected uniformly, the degree of correction varies among the individual NOX sensors. Finally, the NOX concentration information and the oxygen concentration information may vary among the individual NOX sensors. Notably, the reason why the pressure dependency of the sensor output or the concentration value varies among the individual NOX sensors is that the magnitude of the diffusion resistance, which acts on oxygen introduced into the first measurement chamber, varies due to changes in production conditions (hereinafter referred to as production variations).
Further, since the second pumping current used for calculating the NOX concentration has an oxygen concentration dependency, in the technique disclosed in Patent Document 3, the second pumping current is corrected based on the oxygen concentration (oxygen concentration calculated on the basis of the first pumping current). However, the gas pressure dependency of the oxygen concentration itself, which is used for making the correction, is not taken into consideration.